High carbon steel wire rods used typically in prestressed concrete wires, suspension bridge cables, and various wire ropes should have high strengths and satisfactory ductility after wire drawing and, in addition, should have good drawability from the viewpoint of productivity. To meet these requirements, a variety of high quality high carbon steel wire rods have been developed.
Typically, Patent Literature (PTL) 1 proposes a technique of improving resistance to hydrogen embrittlement of a wire rod. This technique specifies the contents of Ti in the forms of a nitride, a sulfide, and a carbide in a spring steel wire rod having a low C content (0.35% to 0.65%) and a high Si content (1.5% to 2.5%) and thereby effectively helps the spring steel wire rod to have finer grains and to trap hydrogen, thus improving the resistance to hydrogen embrittlement.
This technique, however, is intended to be applied to spring steels, and the spring steel wire rod before wire drawing may probably have a structure including ferrite and pearlite. The spring steel wire rod therefore has a low tensile strength and not-so-good drawability as compared to high carbon steel wire rods.
Independently, PTL 2 proposes a technique of improving drawability of a wire rod by specifying the area of intragranular transformed upper bainite present in a cross section of the wire rod and the growth size of such intragranular bainite. The bainitic structure, however, has a lower work hardenability than that of pearlite and fails to provide sufficient strengths after wire drawing.